Magnetic resonance (MR) imaging is a known technique with which images of the inside of an examination subject may be generated, e.g., of a patient in medical imaging. For this purpose, the examination subject is positioned in a magnetic resonance apparatus in a comparatively strong static, homogeneous main magnetic field, also known as the B0-Feld, with field strengths of 0.2 tesla to 7 tesla and more, such that the nuclear spins thereof are orientated along the main magnetic field. To trigger nuclear magnetic resonances, high frequency excitation pulses (also known as RF-pulses) are radiated into the examination subject. The nuclear magnetic resonances that are triggered are measured as what is known as k-space data. On the basis thereof, MR images are reconstructed or spectroscopy data is acquired. For the spatial encoding of the magnetic resonance data, rapidly switched magnetic gradient fields are superimposed on the main magnetic field. The magnetic resonance data that has been recorded is digitalized and stored as complex numerical values in a k-space matrix. From the k-space matrix that is occupied by values, a relevant MR image may be reconstructed, for example, by a multi-dimensional Fourier transform.
The desire for faster and faster magnetic resonance images in the clinical environment is currently leading to a renaissance of methods in which magnetic resonance data from different volume regions of the examination subject, in particular therefore from different slices in a stack of slices, may be recorded simultaneously. These methods may be characterized in that, at least during a part of the measurement, transversal magnetization of at least two slices is used at the same time for the imaging process (“slice-multiplexing”; e.g., known in as simultaneous multi-slice (SMS) imaging). In conventional multi-slice imaging, the signal from at least two slices is recorded consecutively or alternately, that is, completely independent of each other, with an accordingly longer measurement time.
Known methods for this purpose are “Hadamard-coding”, methods with simultaneous echo-refocusing, methods with broadband data recording or even methods that use parallel imaging in the slice direction. The latter methods also include, for example, the blipped Controlled Aliasing in Parallel Imaging (CAIPI) technique, as described by Setsompop et al. in “Blipped-Controlled Aliasing in Parallel Imaging for Simultaneous Multislice Echo Planar Imaging with Reduced g-Factor Penalty”, Magnetic Resonance in Medicine 67, 2012, p. 1210-1224.
U.S. Patent Application Publication No. 2016/0313433 A1 and U.S. Patent Application Publication No. 2015/0115958 A1 disclose methods for simultaneous multi-slice measurement.
In such slice-multiplexing methods, what is known as a multiband high frequency pulse is used in order to excite or otherwise manipulate two or a plurality of slices at the same time, for example, to refocus or saturate them. Such a multiband high frequency pulse is, for example, a multiplex of individual single-slice high frequency pulses, which would be used for the manipulation of the slices that are to be manipulated at the same time. In order to be able to separate the resulting magnetic resonance signals from the various slices, each of the individual high frequency pulses is imprinted with a different phase, (e.g., prior to multiplexing), by adding a linear phase shift, for example, as a result of which the slices are shifted with respect to one another in the location domain. Through multiplexing, one obtains, for example, a basic band-modulated multiband high frequency pulse by adding the shapes of the pulses in the individual single-slice high frequency pulses.
As described, for example, in the article by Setsompop et al. cited above, the g-factor-related disadvantages may be reduced by shifts between the slices, by using gradient blips for instance or by modulating the phases of the individual high frequency pulses. As likewise disclosed in the cited article by Setsompop et al., the signals from the slices that have been excited or otherwise manipulated at the same time may first be combined like signals from only one slice in order for them to then be separated in the subsequent processing by a slice GeneRalized Auto-calibrating Partial Parallel Acquisition (GRAPPA) method.
In the prior art, it has also already been suggested in the context of slice-multiplexing to excite or manipulate in a different way different slices that are meant to be scanned at the same time, such that different contrasts emerge. In this context, U.S. Patent Application Publication No. 2017/0108567 A1 proposes a method for simultaneous multi-contrast recording in SMS imaging, in which with “inversion recovery”, (IR) images may be acquired at the same time as non-IR images, by applying a single band inversion pulse to only one of the slices.
Likewise, with regard to saturation of certain types of spin, the Larmor frequencies of which differ due to a chemical shift, there are already relevant suggestions in existence. The types of spins possibly considered are spins of fat-bound protons (“fat spins”) and spins of water-bound protons (“water spins”). A traditional measure adopted in this context is “fat saturation”. In this context, the subsequently published U.S. Patent Application Publication No. 2018/0024214 discloses combining a binomial pulse for water excitation for a slice with a conventional excitation pulse for the other slice in order to acquire one slice with and one slice without fat saturation. The subsequently published U.S. Patent Application Publication No. 2018/0074146 proposes a method of spatial fat-suppression in multi-contrast SMS imaging, in which a pulse sequence acting on only one slice with a binomial pulse for fat excitation and a dephasing gradient are applied upstream of the further high frequency pulses in order to achieve fat saturation in said slice. The problem here is that the fat saturation may be inadequate in regions with strong B0 distortions. This is because, in those regions, the spins experience stronger or weaker dephasing because the gradient moment that is in fact acting on the spins is generated by the combination of the bipolar gradients and of the deviations in the main magnetic field.
Methods based on magnetic resonance, such as tomographic imaging (e.g., magnetic resonance tomography (MRT)) and spectroscopy (e.g., magnetic resonance spectroscopy (MRS)), may need “benign” physical environmental conditions in order to guarantee an optimum quality of the data acquired. For example, this applies to at least one of the criteria including spatial homogeneity, temporal stability, and the absolute accuracy of the magnetic fields relevant to MR methods (B0, the stationary main magnetic field, and B1, the high frequency magnetic field).
Already known methods with which deviations from ideal environmental conditions may be at least partly compensated for include both system-specific adjustments that seek to correct the given parameters of the magnetic resonance apparatus used, such as for example, eddy current-induced dynamic field distortions or likewise gradient sensitivities, and also examination subject-specific adjustments that attempt to compensate for the changes caused by putting the examination subject, (e.g., a patient), into the measurement volume of the magnetic resonance apparatus, such as susceptibility-related static field distortions or spatial variations in the high frequency field, for instance. To compensate for non-ideal environmental conditions, the affected parameters of the magnetic resonance sequences may be adjusted. In particular, parameters that come into consideration here are the respective central excitation frequency (for example, for an improved fat-suppression and/or a reduced EPI image shift), shimming of the B0 field in the first order (for example, for more homogeneous fat-suppression and/or an improved signal-noise ratio (SNR)), a respective electric voltage of each of the high frequency transmission units (for example, for an improved SNR) and/or B1-shimming (for example, for a more homogeneous SNR). Such examination subject-specific parameters may be derived, for example, from B0 field maps or B1 field maps that have been drawn beforehand.
An adjustment of these recording parameters was first described only for coherent volumes and not, for example, for unconnected slices such as those that are excited or manipulated at the same time in slice-multiplexing. Because the slices to be manipulated at the same time in the slice-multiplexing method are normally arranged as far apart from each other as possible in order to make it easier to separate the slices into signals later, with these methods, an optimization volume in which deviations in the environmental conditions may be corrected therefore includes either the entire stack of slices or at least the envelope of the slices that are to be manipulated at the same time. The parameters included therein are therefore only adjusted on average for the optimization volume and may even be randomly unsuitable for the slices that are actually affected. In particular with measurements on regions of the examination subject with spatially rapidly varying environmental conditions, such as in the region of the patient's head, such adjustments of recording parameters averaged out over fairly large volumes may even lead to the result worsening.
The subsequently published German Patent Application Publication No. DE 10 2016 214 088.4, which has been incorporated in its entirety into the disclosure content of the present description, discloses in this context a method for the slice-specific adjustment of RF pulses in recordings of magnetic resonance data relating to an examination subject with the aid of a slice-multiplexing method in which, for each slice that is to be scanned at the same time, single-slice RF pulse parameters are determined on the basis of the slice position, the single-slice RF pulse parameters are corrected on the basis of at least one examination subject-specific parameter map (e.g., B0 map and/or B1 map), which in each case shows the spatial distribution of one system parameter in the examination subject and of the slice position are corrected and a multi-band RF pulse for the manipulation of the slices to be scanned at the same time is generated on the basis of the corrected single-slice RF pulse parameters. Here, parameters to be adjusted relate to the central-excitation frequency, an amplitude-scaling factor (e.g., transmitter voltage) and/or B1 shim parameters. A correction relating to terms of the first order in the main magnetic field is not possible by the high frequency pulses, so that only the aforementioned averaged correction that considers all the slices measured together is suggested there.
This is particularly problematic with regard to different contrasts and the approaches to fat saturation described in the aforementioned because it has transpired that B0 field deviations of the first order are the main reason for a non-homogeneous fat saturation and other inhomogeneous contrasts.